Field of the Invention
The present invention relates to a driving force transmission apparatus configured to transmit and cut driving force from a driving force source, and an image forming apparatus including the driving force transmission apparatus.
Description of the Related Art
Hitherto, there has been known an image forming apparatus using an electrophotographic technique. In such an image forming apparatus, there are arranged rotary members such as a photosensitive drum, a transfer roller, and a developing roller. Those rotary members are rotated by a driving force source such as a motor. Further, there is a case where an image forming apparatus includes a clutch configured to transmit and cut the driving force from the driving force source.
According to a technology described in Japanese Patent Application Laid-Open No. 2015-18040, an image forming apparatus includes a driving force transmission mechanism configured to transmit driving force from a motor to a driving roller. The driving roller is configured to rotationally move an intermediate transfer belt. Further, the image forming apparatus includes a clutch (clutch B1) configured to transmit and cut driving force from the motor on a driving force transmission path extending from the motor to the driving roller. This clutch includes a first gear (gear 7) configured to be rotated by driving force from the driving force source, and a second gear (gear 8b) configured to be rotated through engagement with the first gear. Further, the clutch includes a third gear (gear 8d), which is arranged coaxially with the second gear and configured to be rotated integrally with the second gear, and a fourth gear (gear 9) configured to be rotated through engagement with the third gear. The fourth gear is connected to a driven member through a transmission mechanism, and driving force of the driving force source is transmitted to the driven member.
Further, the second gear has a toothless portion (portion missing a tooth). When the toothless portion of the second gear is opposed to teeth of the first gear, the first gear and the second gear are prevented from engaging with each other, thereby preventing driving force from being transmitted from the first gear to the second gear. Then, under a state in which the toothless portion of the second gear and the first gear are opposed to each other, a locking member locks the second gear to thereby stop rotation of the second gear. With this, according to the technology described in Japanese Patent Application Laid-Open No. 2015-18040, transmission of driving force from the first gear to the second gear is cut. Further, when the locking member cancels locking with respect to the second gear, the second gear can be engaged with the first gear again. Thus, driving force is transmitted from the first gear to the second gear. In such a manner, driving force from the motor can be transmitted and cut.
According to the technology described in Japanese Patent Application Laid-Open No. 2015-18040, as described above, the clutch includes the third gear arranged coaxially with the second gear. The third gear is connected to the second gear, and hence is rotated integrally with the second gear. Further, the third gear has a convex arc surface, and the fourth gear has a concave arc surface. According to the technology described in Japanese Patent Application Laid-Open No. 2015-18040, the second gear is held so that the toothless portion of the second gear and the first gear are opposed to each other with an elastic force of a spring (torsion spring 11). Further, the elastic force of the spring causes the second gear to rotate by a slight angle so that the second gear and the first gear are engaged with each other again.
At this time, the convex arc surface of the third gear and the concave arc surface of the fourth gear are held in contact with each other, and hence teeth of the third gear and teeth of the fourth gear are prevented from being engaged with each other. Thus, driving force is prevented from being transmitted from the third gear to the fourth gear, and hence the fourth gear is prevented from being rotated. With this, the second gear can be rotated with the elastic force of the spring without rotation of the driven member, thereby being capable of reducing the elastic force of the spring required for rotation of the second gear.
However, according to the technology described in Japanese Patent Application Laid-Open No. 2015-18040, when driving force is transmitted from the fourth gear to the driven member, deformation or torsion may occur in a transmission member (for example, a shaft) configured to transmit the driving force from the fourth gear to the driven member. In a case where the arc surface of the third gear and the arc surface of the fourth gear are held in contact with each other, the teeth of the third gear and the teeth of the fourth gear are prevented from being engaged with each other. Thus, torque which may be generated by torsion in the transmission member may cause the fourth gear to rotate in a reverse direction. With this, a contact pressure between the convex arc surface of the third gear and the concave arc surface of the fourth gear may be increased.
Thus, a larger force may be required in order to rotate the third gear. Therefore, in order to rotate the second gear through the third gear with the elastic force of the spring, the elastic force of the spring needs to be increased. In a case where the torque exerted on the transmission member is large, the related-art spring cannot cause the second gear to rotate.
However, in a case where the elastic force of the spring is to be increased, a material and a shape of the member configured to hold the spring need to be the material and the shape which may offer tolerance against the elastic force of the spring. Therefore, there is a fear in that an image forming apparatus is increased in size, or a fear in that manufacturing cost for the image forming apparatus is increased. Further, the second gear is rotated with a large elastic force, and hence there is a fear in that sound of operation during rotation of the second gear may become larger.